Are most famous poker players compulsive gamblers?

Well, it depends how you define 'gambling'.

There is no question that the most successful winning poker players need to be able to gamble insane sums of money to win like they do.
The giants of poker don't get there by playing entirely tight games.

As far as the downward issue of gambling (Sports, Horses, Craps, etc...), that's an individual thing.
As best I can gather, Barry Greenstein is a 'gambler' but is very prudent with his money.
Conversely, TJ Cloutier isn't a very wealthy man at all even though he is one of the winningest players of all time due to his gambling habits with craps.

And then there's Stu Ungar...

Personally speaking, I realize i'll never be able to sit in a 4000/8000 game just because I have too much respect for what that money represents.
I guess in that sense, i'm not a 'gambler'.
But in a tournament, am I willing to push it all in if I think the other guy is on something soft and I have a bajillion draws that would eat him?
Yeah.